Research And Realization Of The Automatic Impedance Matching Method About Radio Frequency Source

The RF power source has been widely used in vacuum coating,semiconductor etching and plasma induction heating etc.Mostly the frequency of RF source used in industry is 13.56 MHz,and the resistance RS is 50?.However the load impedance is usually not equal to RS.Then if the RF source and load is connected directly,there will be mismatch of varying degrees.Impedance mismatch will cause power reflection on the transmission line,make the energy can not be received entirely by the load and reduce transmission efficiency.If the degree of mismatch is deep,it will cause the failure of the process-cycle.Therefore,it is necessary to set up a matching network between the RF source(or the end of the transmission line)and the load.With the change of working-time and environment factors,load impedance changes in a certain range.In order to get the maximum transmission efficiency,the change of load impedance must be checked constantly and adjust the parameters of the matching network.So the research of automatic impedance matching device is very important.According to the characteristic of change in the manufacturing practice and the changing rule of load impedance on smith chart,? type matching network is designed.The network is composed of a tuning inductor and two adjustable capacitors.The component parameters of the matching network are determined by the load variation range and working conditions.According to the working conditions,the impedance matching network is designed and made.The maximum peak working voltage of the RF source is 200 V and the maximum peak working current is 4A,by rogowski coil which is better than the conventional current transformer in terms of performance,the current signal in the transmission line is detected in the real-time.The ratio is 1:10.Sampling element is selected based on satisfying the conditions of self-integral.Voltage signal in the transmission line is detected by a capacitive divider in the real-time and achieved a certain ratio.The detected voltage and current is input to the amplitude and phase transform circuit of AD8302.Then the amplitude ratio and phase difference can be get.After analog-to-digital conversion,the signal is input into STM32 and calculate the impedance at any time.According to the characteristics of impedance and admittance in Smith chart.PID algorithm is used to control the steering angle of the stepper motor and thus.A new closed loop control system can be realized.The objective of control is to make the circuit matching in 3s,and make the error within 5%.